The automotive industry has sought for a seat back hinge that locks inertially and in which the threshold of sensitivity to inertial imbalance is selectively adjustable and that is sufficiently compact and efficient as to avoid addition of mechanism weight, added space, and that easily self unlocks when the inertially sensed crisis (braking or impact) is over. Economy is also a principal factor in finding a suitable inertial seat back hinge. For example, it is desirable to be able to swing the hinged seats downwardly when the vehicle is stopped or in normal non-decelerating motion without the requirement of manipulating locks and latches and so that the hinge is at all times prepared to lock when a crisis or emergency is inertially sensed.
The prior art includes latches which must be tripped manually and it includes pendulum locks that are both bulky and complex. Some hinges are specially actuated by complex mechanical linkages. Such devices are represented in the U.S. Pat. Nos. 3,549,202, 3,628,831 and 3,674,307 and are distinguishable from the present structure.